When atoms share more than one pair of electrons they form multiple bonds. This dramatically affects bond strength and bond length.
| Bond Type | Shared Pairs | Example | Bond Length | Bond Enthalpy |
|---|---|---|---|---|
| Single (C-C) | 1 | Ethane C₂H₆ | 154 pm | 346 kJ mol⁻¹ |
| Double (C=C) | 2 | Ethene C₂H₄ | 134 pm | 614 kJ mol⁻¹ |
| Triple (C≡C) | 3 | Ethyne C₂H₂ | 120 pm | 839 kJ mol⁻¹ |
Bond Length
As bond order increases, bond length decreases. More shared electron density pulls the nuclei closer together.
Bond Strength (Enthalpy)
As bond order increases, bond enthalpy increases. More shared electrons = stronger electrostatic attraction to both nuclei.
The Inverse Relationship
Bond order ↑ → Bond length ↓ → Bond strength ↑
This is an inverse relationship between length and strength that appears frequently in Paper 1 multiple-choice questions.
Bond Strength is NOT Simply Additive
A double bond is not twice as strong as a single bond. The second pair of electrons (pi bond) overlaps laterally, which is less effective than the first (sigma bond). Similarly, a triple bond is not three times as strong.
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